Phenomena of both late-time and early Universe cosmology, such as dark energy, electro-weak symmetry breaking of the SM Higgs sector, and inflation can enable us to test candidate theories of quantum gravity such as string theory.
The turning point of connecting cosmology to string theory often consists in realizing solutions with positive vacuum energy in string theory, driving ‘accelerated’ cosmic expansion.
We discuss a generic 'reaction' of string theory when using it to describe positive vacuum energy as a source of cosmic acceleration: Backreaction of a string vacuum to positive vacuum energy generically leads to 'flattening' -- the backreacted vacuum energy rises more slowly with its effective control parameter then expected at tree level. We first show examples of inflation in string theory showing this phenomenon, which will be crucial in the future to decipher broad possible predictions from string theory for inflation. We then argue that the backreaction can be come crucially important for describing dark energy with string theory as well: The volume modulus in the simplest KKLT vacua with a single gaugino condensate can flatten the anti-D3 uplift sufficiently to prevent it from reaching dS. We finally discuss the implications of this flattening backreaction effect on our ability to find the landscape within the swampland when it comes to future research connecting string theory to observations.